Other factors that raise the risk include severe pre-deployment symptoms of post-traumatic stress and high combat intensity, researchers report.

But even after taking those factors and past brain injury into account, the study authors concluded that a new traumatic brain injury during a veteran's most recent deployment was the strongest predictor of PTSD symptoms after the deployment.

The study by Kate Yurgil, of the Veterans Affairs San Diego Healthcare System, and colleagues was published online Dec. 11 in JAMA Psychiatry.

Each year, as many as 1.7 million Americans sustain a traumatic brain injury, according to study background information.

A traumatic brain injury occurs when the head violently impacts another object, or an object penetrates the skull, reaching the brain, according to the U.S. National Institute of Neurological Disorders and Stroke.

War-related traumatic brain injuries are common. The use of improvised explosive devices (IEDs), rocket-propelled grenades and land mines in the Iraq and Afghanistan wars are the main contributors to deployment-related traumatic brain injuries today. More than half are caused by IEDs, the study authors noted.

Previous research has suggested that experiencing a traumatic brain injury increases the risk of PTSD. The disorder can occur after someone experiences a traumatic event. Such events put the body and mind in a high-alert state because you feel that you or someone else is in danger.

For some people, the stress related to the traumatic event doesn't go away. They may relive the event over and over again, or they may avoid people or situations that remind them of the event. They may also feel jittery and always on alert, according to the U.S. Department of Veterans Affairs.

Many people with traumatic brain injury also report having symptoms of PTSD. It's been unclear, however, whether the experience leading up to the injury caused the post-traumatic stress symptoms, or if the injury itself caused an increase in PTSD symptoms.

The data came from a larger study following Marines over time. The current study looked at June 2008 to May 2012.

The 1,648 Marines included in the study conducted interviews one month before a seven-month deployment to Iraq or Afghanistan, and a second interview three to six months after returning home.

Before deployment, about 57 percent of the Marines reported having a previous traumatic brain injury.

Of that high number of Marines with a previous brain injury being redeployed, Vincent McGowan, president of the United War Veterans Council, said it's likely that most of these Marines requested redeployment even though they had experienced a previous brain injury.

"Most people want to live and feel productive. Part of healing is feeling that you can be independent," he said.

During deployment, nearly 20 percent of the Marines experienced a new traumatic brain injury. Most of these injuries -- 87 percent -- were classified as mild, according to the study.

Of the 287 Marines who reported post-traumatic amnesia, for the majority, the amnesia lasted less than 24 hours, the study noted. Most of those who lost consciousness due to their injury did so for less than 30 minutes.

The researchers found that pre-deployment PTSD symptoms and high combat intensity slightly increased the risk of post-deployment PTSD. But, mild traumatic brain injury increased the risk of PTSD by 23 percent. Meanwhile, a moderate to severe traumatic brain injury upped the odds of PTSD by 71 percent.

For Marines who had less severe pre-deployment PTSD symptoms, a traumatic brain injury nearly doubled the risk of PTSD, according to the study.

"This is an important study that shows an even greater effect between a brain injury and psychological trauma than might have been expected," said Rachel Yehuda, a professor of psychiatry and director of the traumatic stress studies division at the Icahn School of Medicine at Mount Sinai, in New York City.

"When you're in combat, it's good to be on alert. When you come home, if you're not exposed to an ongoing threat, stress symptoms should get milder over time," Yehuda said. "But, it makes sense that if you have a brain injury, it may be harder to recover because the brain may continue to feel like there is an ongoing threat."

She said it's important for veterans coming home from war with a traumatic brain injury to know that they're at an increased risk of PTSD, and that it's important to seek help if they need it.

For his part, McGowan said it's important to use VA care for any service-related injury or disability so that veterans have access to ongoing care.